The present invention relates to methods to measure the transit time values for pulses in time domain waveform data. Examples will be presented using Time-Domain Terahertz data to determine sample properties. Terahertz electromagnetic radiation is potentially useful in many industrial measurement applications. In TD-THz, essentially single cycle pulses (approximately 1 ps width, FIG. 1) of radiation are synchronously generated and detected. This synchronous method results in high fidelity measurement of the radiation's electric field strength over a waveform time window. This width of this window can vary over a wide range depending on the instrumentation used. As THz radiation pulses are very brief in time, they will contain an extremely wide band of frequencies (10 GHz up to 50 THz).
Once a TD-THz pulse has interacted with a sample, a number of useful measurements can be extracted from acquired time-domain data. Possible measurements include, but are not limited to, sample mass, thickness, density, refractive index, density and surface variations, and spectroscopy (e.g. moisture content, polymorph identification). In
FIG. 2 illustrates a terahertz transmitter 10 and terahertz receiver 12. TD-THz, the changes in the THz pulses after they have interacted with material are recorded in a time-domain waveform. For example, as the pulse transmits through a sample 14, the pulse's arrival at the receiver will attenuated and delayed compared to the transmission of the same pulse through an air path (FIG. 2). The amount of the pulse delay is determined by the material's group refractive index value and the amount of mass in the sample beam. The attenuation of the pulse is also dependent on material's refractive index (Fresnel reflection loss), the scattering of radiation with the sample and the attenuation of the pulse's frequencies by the material.
In the top schematic, a THz pulse travels through air with minimum time-of-flight and no loss in amplitude. The addition of essentially transparent solid materials (e.g., plastics, paper, and cloth), in the THz beam path (lower left) will result in a longer time-of-flight for the pulse. The increased time-of-flight will be proportional to the mass and index of refraction of the material. In the bottom right schematic, scattering or absorbing media (such as foam or water laden cloth) will reduce the pulse amplitude in addition to generating a time-of-flight pulse delay.
Many measurements can be made with reflections of the TD-THz pulses off the sample (FIG. 3). This figure illustrates a subset of possible interactions and thus sample properties that can be measured. The consistent need for all measurements is the precision determination of the transit time value for the TD-THz pulse(s).
An example measurement would be sample thickness measurement. This measurement could be made in either transmission or reflection optical geometries. In transmission, the delay of the THz pulse by the sample 14 can be used to measure thickness (FIG. 4). In FIG. 4, Line 16 represents no sample. Line 18 represents a thin sample. Line 20 represents a thick sample. This method requires determining the time position of peaks from two time-domain waveforms that were acquired at two different times (i.e., sample in and out of the beam). This method can result in offset or scaling errors if the position of either peak is shifted due to instrumental or environmental conditions.
Alternatively, THz pulses will reflect some energy at any interface (e.g., Fresnel reflection). Referring to FIG. 5, a multipass sample chamber 21 is illustrated. Reflections using mirrors 22 and 24 from the front and rear surface of a sample can be observed as shown in FIG. 6. The time delay between these two reflection peaks is determined by the mass and the refractive index of the material. Thus, it is possible to measure the sample's mass, thickness and/or density of a sample from a single time-domain waveform. Measurements made in this manner will exhibit reduced offset or timing slope errors.
Multipassing the THz pulses through the sample would increase the observed time delay without changing the imprecision of the pulse time measurement (as long a sufficient Signal-to-Noise is maintained). This concept is illustrated in FIG. 4. This method would increase the overall sample thickness measurement precision.
An interesting aspect of reflection waveforms is the polarity of the waveform pulses. TD-THz measures the direct electric field, thus the polarity of the pulse does indicate the electric field polarity. In transmission measurements, the presence of a sample does not affect the pulse polarity. However, for reflection measurements, the pulse will flip polarity when reflecting off a low to high refractive index or metal interface. That is why the first pulse in the reflection waveform of FIG. 6 (air-to-sample) is flipped in polarity. The strength of reflection is dependent, among other factors, on the difference in refractive index between the two materials. This information can be used to determine the delta refractive index change, including the sign of the delta, of the two materials across the interface.